Association of breast milk betaine and related metabolites with neurodevelopment in 6-month-old infants: a prospective birth cohort study

<i>Background and aims</i>: Currently, there is a paucity of systematic research investigating the association between betaine and its related metabolites in breast milk and infant neurodevelopment. To address this gap, this study aimed to examine the relationship between concentrations of betaine, choline, trimethylamine <i>N</i>-oxide (TMAO), and L-carnitine in breast milk and infant neurodevelopment. <i>Methods</i>: A prospective birth cohort study was conducted, enrolling 158 mother-infant pairs. Breast milk samples were collected at 42 days postpartum, and concentrations of the target metabolites were quantified using high-performance liquid chromatography-tandem mass spectrometry. Infant neurodevelopment was evaluated at 6 months of age using the "Child Developmental Scale (0-6 Years)", from which developmental quotients (DQ) across multiple domains were derived. Generalized linear models and Bayesian Kernel Machine Regression (BKMR) were employed to assess both individual and combined effects of the four metabolites on infant development. <i>Results</i>: Breast milk betaine levels were significantly positively associated with DQ scores in gross motor, fine motor, adaptability, and social behavior domains. Choline concentrations showed positive association with fine motor and adaptability DQ. In contrast, higher TMAO and L-carnitine levels were linked to lower adaptability and gross motor DQ, respectively. BKMR analysis identified betaine as the predominant contributor to overall developmental DQ among the four metabolites, with the strongest influence observed for the fine motor domain (posterior inclusion probability = 0.880). No significant pairwise interactions between metabolites were detected. <i>Conclusion</i>: This study offers comprehensive evidence regarding the relationships between breast milk levels of betaine, choline, TMAO, and L-carnitine and infant neurodevelopment at 6 months of age. These findings may inform personalized nutritional strategies during lactation to support optimal infant neurodevelopment. Clinical Trial Registry number: ChCTR2200055202.